Martian and Lunar Meteorites: Styles of Aqueous Alteration

Introduction: With the recent return of remote sensing evidence for aqueous alteration on Mars in the form of evaporates and other secondary minerals (MERs [e.g., 1] and Mars Express [e.g., 2]), understanding the differences between weathering processes on Mars and Earth becomes imperative. Martian meteorites provide the only available source of detailed geochemical analyses of Martian materials besides data returned by remote sensing. Alteration has been shown to occur quickly during a meteorite's residence on Earth in both falls and finds and in both 'hot' and 'cold' deserts [e.g, 3]. Many studies of alteration in chondrites are reported in the literature [e.g., 3-4]. Chondrites, in addition to any terrestrial alteration, can also be aqueously processed on their parent body [e.g. 5]. Previous studies have identified several alteration phases in Martian meteorites [e.g., 6-10]. Like chon-drites, which experience parent-body alteration, care must be taken to separate terrestrial and Martain aque-ous alteration products, and a set of criteria are already in place to accomplish this task (see [7] for a review). Alteration in lunar meteorites has been largely overlooked (except [11], who studied terrestrial alteration of 2 lunar anorthositic breccias), probably because it does not represent alteration on the parent body. Instead, since the Moon does not experience aqueous processes, all of the alteration is, therefore, terrestrial. This eliminates any uncertainty about the source of the alteration. And because of the similiarity of many terrestrial environments (e.g., the Dry Valleys of Antarc-tica [7]) to potential environments on Mars, we can use the terrestrial alteration of lunar meteorites (esp. lunar basalts) as another analog to aqueous alteration on Mars. Here, we investigate the different styles of alteration in Martian meteorites and lunar meteorites to assess the importance of the parent material in style of alteration experienced. And we assess the value of studying alteration of lunar meteorites as a potential analog for weathering on Mars. Methods: This is a preliminary study of alteration in various types of meteorites, and for this preliminary phase of study, SEM backscatter imagery and SEM EDS semi-quantitative point analyses and X-ray elemental maps have been aquired for each meteorite: 2 nakhlites and 3 lunar meteorites (2 basalts and 1 highlands regolith breccia). In addition, microprobe EMPA and a modal mineralogy map are available for Martian meteorite MIL 03346 [12]. In future studies, we will add more meteorites to the survey as well as additional analytical methods.